This invention relates to a method and machine for controlling picture color in a digital manner, used in a picture reproducing machine such as a color scanner, a color facsimile, or the like.
In a conventional picture reproducing machine such as a color scanner, a color facsimile, or the like, a color control operation such as a masking, a color correction, and so forth, has been carried out in an analog manner by operating electronically picture signals obtained by scanning an original picture. This method has good stability, reliability, reproducibility, and the like, rather than a photographic method.
However, thereafter, more strict conditions of the stability, the reliability, the reproducibility, and so forth, have been required, but a conventional analog operational circuit could not meet these requirements because integrated operational amplifiers, registers, potensiometers, and many other elements of the analog operational circuit depend on temperatures and times used. Thus, after a long period time, while they are used, the stability, the reliability, the reproducibility, and the like, of these elements are deteriorated.
In order to overcome these problems, when the operational circuit are composed, the best grade of elements are used and temperature compensation circuits are added. However, this results in the complicated operational circuit, and accordingly, in general, the drop of the reliability, and high cost.
Then, in order to remove the stability, the reliability, the reproducibility, and so forth, a digital operational method has been proposed. In this method, the operation of the signals is performed in the real time processing at high speed, and the color correction is carried out by transforming, not calculating, quickly coordinates of input color separation signals R, G and B of additive primary colors such as red, green and blue into those of output color separation signals Y, M and C of the subtractive primary colors such as yellow, magenta and cyan corresponding thereto.
If each red, green and blue range is divided, for example, into 2.sup.8 tone steps, or each color is coded by eight bits, the capacity corresponding to altogether 2.sup.24 steps for a combination of three colors must be required, and consequently such a coordinates transformation method requires a memory having a large capacity. This means high cost, and thus is not practicable.
In this method, the coordinates transformation is performed by a three-dimentional memory table wherein the combination of three color recording digital signals Y, M and C are stored along three axes and are read out of the table by addressing by means of the combination of three color picture digital signals R, G and B corresponding thereto. This method is operated at the high speed, but, in practice, the capacity of the memory table is restricted.
Then, in order to reduce the capacity of the memory, an interpolation method has been applied. In this case, each red, green and blue range is divided more roughly into the tone steps in the three-dimentional coordinates and inermediate values between the steps are interpolated from the adjacent stored values corresponding to the combinations of the recording signals Y, M and C, which are read out of the memory of the combinations of the picture signals R, G and B corresponding thereto.
However, in this case, the relation between the picture and the recording signals is represented by a quadratic equation, but then, in practice, the interpolation is carried out linearly in the approximate manner. Accordingly, the errors of the approximate values interpolated are varried depending on the quadratic equation and are often beyond the acceptable limit range. In order to perform a faithful interpolation operation is required a complex interpolation method which takes much time. Thus, it is almost imposible to carry out this complex interpolation method in the real time processing.
On the other hand, the essential conditions for the color correction, when the picture reproducing machine such as a color scanner is operated, are itemized in the followings.
1. The color correction conditions are settled readily; PA0 2. The color correction conditions are minimized; PA0 3. Each color correction condition is independently settled; PA0 4. The color correction conditions are obvious as compared with their standard values; PA0 5. The color correction conditions are expressed easily in a specification; PA0 6. The color correction conditions are maintained for a long period of time; PA0 7. The same color correction conditions are obtained after a long period of operational time; PA0 8. The color correction conditions are readily recorded in a recording medium such as a tape or a card; and PA0 9. The data of the color correction conditions which has been obtained experientially by operating the color scanner are utilized continuously, and the other data will be joined in the previous data.
These items should be satisfied in the digital method. However, the conventional digital method could not satisfy all of these items.
For instance, in the conventional method utilizing the three-dimentional memory table, each color correction condition cannot be determined independently, and is closely related to the other color correction conditions. Therefore, if the part of the conditions is replaced, all of the table must be changed. This means a large number of tables are required for all of the possibilities of the color correction conditions. Further, when the interpolation is carried out, the color correction conditions are reflected on it. Thus, this is not practicable and high cost.
This digital processing is substantially a coordinates transformation from the combination of three color picture signals R, G and B to the combination of three color recording signals Y, M and C by a table, as described above, and the color correction conditions such as hue, saturation, brightness, color balance, and so forth, are related closely one another. Therefore, it is difficult to indicate the corrected amounts of these color correction conditions in the same manner as a conventional analog method.
From the above description, it is understood that the conventional digital color scanner cannot satisfy the above items 1, 3, 4, 5 and 9.
As regards the item 9, in particular, this is not an essential function of the color scanner, but is an important condition which decides whether the data obtained can be utilized continuously. The conventional digital method cannot satisfy this condition, which is a large inconvenience.
Further, in the conventional three-dimentional coordinates transformation process, if each picture signal R, G and B is coded by a binary code having 8 bits, each combination of three picture signals R, G and B corresponds to a binary code having 24 bits, whose capacity is 2.sup.24 imformation.
The color corresponding to the combination of three picture signals R, G and B is represented by brightness, saturation and hue, as is well-known, and the brightness has the maximum resolving power. In the conventional digital method, the brightness, saturation and hue are expressed by binary codes, each having 8 bits. However, the brightness and the saturation are represented by a pure color component and a gray color component including an equivalent gray density component, and hence the saturation possesses a redundancy, viz. the code of the saturation includes redundant bits. Meanwhile, in practice, when the resolving power of the hue may be reduced as compared with that of the brightness, the influences of the colors of the reproduction pictures may be ignored.
Accordingly, in the data processing, the sampling steps of the saturation and the hue may be pressurized, or settled more roughly, and thus these two may be represented by binary codes having 6 bits, i.e. altogether 2.sup.20 information for a combination of three color picture signals R, G and B is required.